TITLE: GCN CIRCULAR NUMBER: 38601 SUBJECT: GRB 241218A: Swift detection of a burst DATE: 24/12/18 00:58:22 GMT FROM: Boris Sbarufatti at INAF-OAB boris.sbarufatti@inaf.it
M. A. Williams (PSU), R. Gupta (NASA GSFC), N. P. M. Kuin (UCL-MSSL), B. Sbarufatti (INAF-OAB) and A. Tohuvavohu (Caltech) report on behalf of the Neil Gehrels Swift Observatory Team:
At 00:08:51 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 241218A (trigger=1275005). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 185.682, +29.991 which is RA(J2000) = 12h 22m 44s Dec(J2000) = +29d 59' 27" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a complex structure with a duration of about 25 sec. The peak count rate was ~896 counts/sec (15-350 keV), at ~0 sec after the trigger.
The XRT began observing the field at 00:11:15.6 UT, 144.5 seconds after the BAT trigger. The position determined from promptly downlinked data differs significantly from the on-board position, suggesting that the XRT may have centroided on a cosmic ray; the initial XRT position notice should be treated with caution. Using promptly downlinked data we find a fading, uncatalogued X-ray source with an enhanced position: RA, Dec 185.68146, 29.97303 which is equivalent to: RA(J2000) = 12h 22m 43.55s Dec(J2000) = +29d 58' 22.9" with an uncertainty of 1.9 arcseconds (radius, 90% containment). This location is 64 arcseconds from the BAT onboard position, within the BAT error circle. This position may be improved as more data are received; the latest position is available at https://www.swift.ac.uk/sper.
A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 2.23 x 10^20 cm^-2 (Willingale et al. 2013).
UVOT took a finding chart exposure of 150 seconds with the White filter starting 151 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of the XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag. The 8'x8' region for the list of sources generated on-board covers 100% of the XRT error circle. The list of sources is typically complete to about 18 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.023.
Burst Advocate for this burst is M. A. Williams (mjw6837 AT psu.edu). Please contact the BA by email if you require additional information regarding Swift followup of this burst. In extremely urgent cases, after trying the Burst Advocate, you can contact the Swift PI by phone (see Swift TOO web site for information: http://www.swift.psu.edu/)
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